1. Field of the Invention
The present invention relates to a novel ice nucleation active microorganism, more specifically, to a novel ice nucleation active Xanthomonas strain and a bacterial ice nucleator comprising the ice nucleation active microorganism which can be applied for food processing and artificial snow making.
2. Description of the Prior Art
Microbial ice nucleation(the initiation of the crystallization of liquid water into solid ice) has been studied since the early 1970's, and it has been discovered that frost and cold-weather injuries in plants are caused by microorganism, not by climate effect(see: Schnell and Vali, Nature, 246(23):21-33(1973)). Also, studies on ice nucleation protein("INP") have followed, since the discovery that the damage is originated from the INP of ice nucleation active microorganism. Thus, it has been found that the INP has an ice-nucleation activity, and microorganisms producing the protein are ephiphytic microorganisms living on the surface of plant leaves.
Bacterial ice-nucleation activity have been researched, chiefly concentrated on the prevention of frost injury to the agricultural crops and its potential application in various technical areas. As a result, microbial strains competing with the ice nucleation active microorganisms or mutants losing the ice-nucleation activity have been developed to prevent frost injury caused by the ice nucleation active microorganisms(see: Lindemann and Suslow, Phytopathol., 77:882-886(1987)). In the application of the ice nucleation active microorganism, it has been widely used as an inducer for artificial snow making.
Though it has been known that water freezes at 0.degree. C., the probability that pure water will nucleate homogenously at 0.degree. C. is, in fact, zero. As tiny droplets of pure water(less than 1 .mu.m in diameter) are decreased in temperature, the probability approaches 1 at -41.degree. C. A temperature of -41.degree. C. is usually accented as the limit to which water can be super-cooled. However, the freezing of water is catalyzed at -2.degree. C. to -4.degree. C. in the presence of INP, since the INP elevates the freezing point of water. In general, several ice nucleation active bacteria are able to catalyze ice formation at temperature as warm as -2.degree. C. That is, though the freezing of water requires super-cooling condition at -41.degree. C., in the presence of INP, super-cooling condition at -2.degree. C. to -4.degree. C. is required, which results in the saving of energy.
The effect of energy saving enables the ice nucleation active microorganism to be industrially applied as a freeze-inducer for preserving food products. Accordingly, the ice nucleation active microorganism can contribute to the preservation of frozen foods, the production of ices, the prevention of loss, the improvement of quality and the saving of energy. Also, the ice nucleation active microorganism can be applied to the freeze-concentration of liquid food and the production of frozen food. In addition, the ice nucleation active microorganism can induce artificial rain during a drought as a stimulant of the artificial rain, and is presently tested for field. In conclusion, it is expected that the ice nucleation active microorganism can be applied to all industrial areas associated with ice nucleation active process.
Particularly, the application of bacterial ice nucleator reduces freezing times and improves the quality of frozen foods, which suggests that there may be profound potential for quality improvement in food industry. Application of ice nucleation active microorganism in food industry has following advantages: First, energy required for freezing of food products is remarkably saved, since freezing occurs at -5.degree. C. or more employing ice nucleation active microorganism while the conventional freeze-concentration or freeze-drying processes require a cooling temperature of -20 to -30.degree. C. Since the freeze-concentration requires much energy for cooling, it has not practically been used regardless of its distinguished merits. However, the use of ice nucleation active microorganism permits wide use of freeze-concentration or freeze-drying due to the effect of energy saving. Secondly, the use of ice nucleation active microorganism in food processing leads to development of goods having a new property. For example, ice nucleation active microorganisms encapsulated in alginate gel, are used for freeze-drying of soy sauce and soybean paste, which makes possible energy saving caused by rapid freezing and easy grinding caused by porous dried structure. Also, through freeze-texturing of egg albumin, etc. employing ice nucleation active microorganism, texture of specific flake shape can be obtained, which has a distinction over the texture conventionally obtained by freeze-texturing. Furthermore, it has been reported that gel made of egg albumin which is freeze-concentrated by the aid of ice nucleation active microorganism has good properties of matter in light of foam formation and stability (see: Michiko, W. et al., J. food Eng., 22:453-473 (1994); Keiko, K. et al., Biol. Biotech. Biochem., 57(5):750-752(1993)).
Besides, studies on the production of foods of high quality by a combined use of ice nucleation active microorganism and ultra-high pressure processing, have been actively carried out. For example, freeze-concentrated milk whose protein components are not denaturated by heat treatment, by employing ice nucleation active microorganism, have been found to form specified gel during pressurization although gelling agent is not added therein. Since the gel thus formed has good color, good brightness and fresh cream-flavor, a dessert of high quality have been made of the gel(see: Rikimaru, H. et al., Agri. Biol. Chem., 53(11) :2935-2939(1989)). Also, strawberry jam having good natural flavor and containing much vitamin C can be produced, compared to the conventional strawberry jam made by heat treatment, when strawberry paste was freeze-concentrated using ice nucleation active microorganism and then pressurized(see: Michiko, W. et al., Agri. Biol. Chem., 55(8):2175-2176(1991)).
On the other hand, ice nucleation active microorganisms have been applied, primarily concentrated on the bacterial strains of Pseudomonas and Erwinia species, in the technical areas covering freeze-drying process, commercially important ice or snow making, and food processing employing the said microorganisms. However, the use of ice nucleation active microorganism belonging to Pseudomonas or Erwinia species can not guarantee safety of food products, and ice-nucleation activity of the said microorganisms has to be improved so that they can be applied as bacterial ice nucleator for food processing.
Accordingly, there are strong reasons for exploring and developing alternative ice nucleation active microorganism, to solve the problems of conventional strains in light of ice-nucleation activity and safety of this particular material in the commercial freezing of food products.